It has been previously described that p21 functions not only as a CDK inhibitor but also as a transcriptional co-repressor in some systems.To investigate the roles of p21 in transcriptional control, we studied the gene expression changes in two human cell systems.Using a human leukemia cell line (K562) with inducible p21 expression and human primary keratinocytes m7 mega truck with adenoviral-mediated p21 expression, we carried out microarray-based gene expression profiling.
We found that p21 rapidly and strongly repressed the mRNA levels of a number of genes involved in cell cycle and mitosis.One of the most strongly down-regulated genes was CCNE2 (cyclin E2 gene).Mutational analysis in K562 cells showed that the N-terminal region of p21 is required for repression of gene expression of CCNE2 and other genes.
Chromatin immunoprecipitation assays indicated that p21 was bound to human CCNE2 and other p21-repressed genes gene in the vicinity of the transcription start site.Moreover, p21 repressed yamaha bbne2 human CCNE2 promoter-luciferase constructs in K562 cells.Bioinformatic analysis revealed that the CDE motif is present in most of the promoters of the p21-regulated genes.
Altogether, the results suggest that p21 exerts a repressive effect on a relevant number of genes controlling S phase and mitosis.Thus, p21 activity as inhibitor of cell cycle progression would be mediated not only by the inhibition of CDKs but also by the transcriptional down-regulation of key genes.